In sheet-fed offset printing machines, the printing plates for the differently colored part images are each mounted on a plate cylinder. Each printing plate is fastened (clamped) in a tension rail by means of an edge assigned to the print start and is then laid around the outer circumference of the cylinder. The printing plate's rear edge assigned to the print end is fastened in turn in a tension rail. Tensioning of the printing plate then takes place.
Since it is desirable for the prints to have a very high degree of accuracy of fit from the very outset in order to reduce waste, the front edge of individual printing plates must be introduced into the corresponding print-start tension rail very carefully. When a plate is inserted into the tension rails within the prescribed accuracy, the plate is referred to as being in-register. In order to achieve the prescribed level of accuracy, the printing plates often have, at the edge assigned to the print start, recesses in the form of punched-out portions which cooperate with corresponding locating pins in the tension rails. However, proper engagement between the recesses and the locating pins is obtained only when the respective printing-plate front edge has been brought to bear on such devices in an exact manner.
EP 0,075,900 B1 discloses an apparatus for checking the in-register clamping of printing plates having sensing pins arranged in the print-start tension rail. The in-register position of the printing plate is detected when the printing-plate edge contacts the sensing pins. This known device provides, in particular, an indicator which is arranged within the cylinder pit and means for adjusting the position of the printing plate in a controlled manner via an adjustable front tension rail.
EP 0,195,848 B1 discloses an apparatus which acts in a similar way. Blocks electrically insulated from the impression cylinder act as stop probes which cooperate with corresponding punchings in the printing plate. Light-emitting diodes integrated therein and a current supply which powers the diodes when the printing plate or the edge of the locating perforation comes into contact with the electrically insulated probes indicate whether the printing plate has been oriented in-register.
However, these previously known devices described briefly above are suitable only for manual drawing on of the printing plate since the operator must read off the respective corresponding optically transmitted signals directly in the cylinder pit or on the measuring sensors themselves while the printing plate is being introduced or while the printing plate is being oriented.
DD 69,382 PS discloses locating pins which are electrically insulated relative to the plate cylinder and which make an electrically closed contact when in contact with the printing plate or an edge of the printing plate. The closed electrical contact is sensed by the printing-machine control via a measurement transducer mounted on the rotatably mounted plate cylinder. Such a device can thus be used for an automated printing-plate feed. However, a disadvantage of this sensing apparatus is that a measurement transducer, consisting, for example, of a slip ring and current collector, has to be provided. This arrangement, however, introduces an additional source of possible machine failure.
DE 3,940,796 C2 discloses an apparatus for automatic changing of printing plates in which the above-mentioned measuring arrangement could be used. A printing plate is introduced from a storage chamber via transport rollers into a tension rail of a plate cylinder which has previously been moved into a specific angular position to receive the plate. A position detector, not specified in any more detail in this publication, detects whether the printing plate assumes a correct bearing within the tension rail assigned to the print start. If a proper position is indeed detected, the devices for closing the tension rail (clamps) are activated.
JP-A 62-22 1541 discloses as a printing-plate introduction aid for feeding printing plates into the print-start tension rail, in which a type of bearing table is placed against the correspondingly positioned plate cylinder. This bearing table has pivotable locating pins which cooperate with corresponding punched-out portions of the printing plate. When these are pivoted away and out of the conveying path of the printing plate, the pre-positioned printing plate falls into the opened print-start tension rail. However, according to this publication, an electrical scanning of the correct bearing of the printing plate within the rail is not provided.